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8
MCQ (Multiple Correct Answer)

Consider a simple circuit shown in figure stands for a variable resistance $R^{\prime} \cdot R^{\prime}$ can vary from $R_0$ to infinity. $r$ is internal resistance of the battery $(r<< R< R$,$) .$

A
Potential drop across $A B$ is nearly constant as $R^{\prime}$ is varied
B
Current through $R^{\prime}$ is nearly a constant as $R^{\prime}$ is varied
C
Current $I$ depends sensitively on $R^{\prime}$
D
$I \geq \frac{V}{r+R}$ always
9
MCQ (Multiple Correct Answer)

Temperature dependence of resistivity $\rho(T)$ of semiconductors, insulators and metals is significantly based on the following factors

A
number of charge carriers can change with temperature $T$
B
time interval between two successive collisions can depend on $T$
C
length of material can be a function of $T$
D
mass of carriers is a function of $T$
10
MCQ (Multiple Correct Answer)

The measurement of an unknown resistance $R$ is to be carried out using Wheatstones bridge as given in the figure below. Two students perform an experiment in two ways. The first students takes $R_2=10 \Omega$ and $R_1=5 \Omega$. The other student takes $R_2=1000 \Omega$ and $R_1=500 \Omega$. In the standard arm, both take $R_3=5 \Omega$. Both find $R=\frac{R_2}{R_1}, R_3=10 \Omega$ within errors.

A
The errors of measurement of the two students are the same
B
Errors of measurement do depend on the accuracy with which $R_2$ and $R_1$ can be measured
C
If the student uses large values of $R_2$ and $R_1$ the currents through the arms will be feeble. This will make determination of null point accurately more difficult
D
Wheatstone bridge is a very accurate instrument and has no errors of measurement
11
MCQ (Multiple Correct Answer)

In a meter bridge, the point $D$ is a neutral point (figure).

A
The meter bridge can have no other neutral. A point for this set of resistances
B
When the jockey contacts a point on meter wire left of $D$, current flows to $B$ from the wire
C
When the jockey contacts a point on the meter wire to the right of $D$, current flows from $B$ to the wire through galvanometer
D
When $R$ is increased, the neutral point shifts to left
12
Subjective

Is the motion of a charge across junction momentum conserving? Why or why not?

Explanation

When an electron approaches a junction, in addition to the uniform electric field $\mathbf{E}$ facing it normally. It keep the drift velocity fixed as drift velocity depend on $E$ by the relation drift velocity

$$v_d=\frac{e E \tau}{m}$$

This result into accumulation of charges on the surface of wires at the junction. These produce additional electric field. These fields change the direction of momentum. Thus, the motion of a charge across junction is not momentum conserving.

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